In a setback for the Federal Communications Commission, a federal court issued a ruling on Monday that may force cable companies to share their high-speed Internet, or broadband, networks with competing Internet access providers.

The decision, issued by a three-judge panel of the Ninth U.S. Circuit Court of Appeals in San Francisco, found that the FCC erred in an earlier ruling that effectively absolved cable companies of any obligation to make their lines accessible to competitors.

The decision was hailed by Internet access providers that sued to get the right to lease those lines and offer competing services over them. They said the court decision would give consumers more choice when shopping for a provider of high-speed Internet access.

"This will help drive prices down and quality of service up. It will drive broadband deployment," said Dave Baker, vice president of law and policy for EarthLink, an Internet access provider and a plaintiff in the suit.

Despite claims of victory by the cable industry's competitors, telecommunications lawyers familiar with the latest case said the implications of the 39-page ruling may not be clear for some time. They said the decision does not specifically require cable companies to lease their lines to competitors. Rather, the court ruled that the FCC was wrong in the way it categorized cable broadband services for regulatory purposes.

In March 2002, the commission ruled that it would regulate cable broadband providers as "information services," a definition that applies to companies that process data. Companies that fall under that definition are subject to much less stringent regulation.

The FCC's approach toward broadband regulation -- for both cable companies and telephone companies -- is to permit the major players to build out their high-speed Internet infrastructure without forcing them to open their networks to competitors. The FCC has said it believes the best way to expand deployment is to give the big companies incentive to invest in new networks.

The appellate court, however, found that cable broadband service providers are in part providing "telecommunications services," a definition that could subject them to the greater obligations of "common carriers" under the federal telecommunications law.

The court indicated in a footnote in the ruling that "the practical result of such a classification is that cable broadband providers would be required to open their lines to competing" Internet service providers.

But that result is by no means certain. In a preliminary decision in February, the FCC said it would not force the telephone companies to lease their high-speed Internet access, or digital subscriber lines, to competitors. The commission could apply that ruling to cable companies as well.

Even before the ruling, some cable companies reached agreements with Internet service providers to lease their high-speed cable lines. For instance,

Comcast Cable, one of the nation's largest cable companies, has agreements to share its network with several national Internet service providers, including America Online and EarthLink, as well as regional providers.